Enhancement of Apple Stress Resistance via Proline Elevation by Sugar Substitutes.

Plants encounter numerous adversities during growth, necessitating the identification of common stress activators to bolster their resistance. However, the current understanding of these activators' mechanisms remains limited. This study identified three anti-stress activators applicable to apple trees, all of which elevate plant proline content to enhance resistance against various adversities. The results showed that the application of these sugar substitutes increased apple proline content by two to three times compared to the untreated group. Even at a lower concentration, these activators triggered plant stress resistance without compromising apple fruit quality. Therefore, these three sugar substitutes can be exogenously sprayed on apple trees to augment proline content and fortify stress resistance. Given their effectiveness and low production cost, these activators possess significant application value. Since they have been widely used in the food industry, they hold potential for broader application in plants, fostering apple industry development.

Low nitrate alleviates iron deficiency by regulating iron homeostasis in apple.

Iron (Fe) is an essential element for plant growth, development and metabolism. Due to its lack of solubility and low bioavailability in soil, Fe levels are usually far below the optimum amount for most plants' growth and development. In apple production, excessive use of nitrogen fertilizer may cause iron chlorosis symptoms in the newly growing leaves, but the regulatory mechanisms underlying this phenomenon are unclear. In this study, low nitrate (NO3- , LN) application alleviated the symptoms of Fe deficiency and promoted lower rhizosphere pH, which was beneficial for root Fe acquisition. At the same time, LN treatment increased citrate and abscisic acid accumulation in roots, which promoted Fe transport from root to shoot and maintained Fe homeostasis. Moreover, qRT-PCR analysis showed that nitrate application caused differential expression of genes related to Fe uptake and transport, as well as transcriptional regulators. In summary, our data reveal that low nitrate alleviated Fe deficiency through multiple pathways, demonstrating a new option for minimizing Fe deficiency by regulating the balance between nutrients.

Nitrogen reduces calcium availability by promoting oxalate biosynthesis in apple leaves.

N and Ca are essential nutrients for apple growth and development. Studies have found that Ca content was not low under high N conditions but was poorly available. However, the underlying physiological mechanism through which N regulates Ca availability remains unclear. In this study, apple plants were supplied with N and Ca to analyse the content, in situ distribution, and forms of Ca using noninvasive micro-test technique, electron probe microanalysis, Fourier transform infrared spectroscopy, and transcriptome analysis. A potential interaction was observed between N and Ca in apple leaves. The application of high N and Ca concentration led to a CaOx content of 12.51 g/kg, representing 93.54% of the total Ca in the apple leaves. Electron probe microanalysis revealed that Ca deposited in the phloem primarily existed as CaOx rhombus-shaped crystals. Additionally, high N positively regulated oxalate accumulation in the leaves, increasing it by 40.79 times compared with low N concentration. Specifically, N induced oxalate synthesis in apple leaves by upregulating the MdICL, MdOXAC, and MdMDH genes, while simultaneously inhibiting degradation through downregulation of the MdAAE3 gene. Transcriptome and correlation analyses further confirmed oxaloacetate as the precursor for the synthesis of CaOx crystals in the apple leaves, which were produced via the 'photosynthesis/glycolysis -oxaloacetate -oxalate -CaOx' pathway. WGCNA identified potential regulators of the CaOx biosynthesis pathway triggered by N. Overall, the results provide insights into the regulation of Ca availability by N in apple leaves and support the development of Ca efficient cultivation technique.

MdBT2 regulates nitrogen-mediated cuticular wax biosynthesis via a MdMYB106-MdCER2L1 signalling pathway in apple.

Cuticular waxes play important roles in plant development and the interaction between plants and their environment. Researches on wax biosynthetic pathways have been reported in several plant species. Also, wax formation is closely related to environmental condition. However, the regulatory mechanism between wax and environmental factors, especially essential mineral elements, is less studied. Here we found that nitrogen (N) played a negative role in the regulation of wax synthesis in apple. We therefore analysed wax content, composition and crystals in BTB-TAZ domain protein 2 (MdBT2) overexpressing and antisense transgenic apple seedlings and found that MdBT2 could downregulate wax biosynthesis. Furthermore, R2R3-MYB transcription factor 16-like protein (MdMYB106) interacted with MdBT2, and MdBT2 mediated its ubiquitination and degradation through the 26S proteasome pathway. Finally, HXXXD-type acyl-transferase ECERIFERUM 2-like1 (MdCER2L1) was confirmed as a downstream target gene of MdMYB106. Our findings reveal an N-mediated apple wax biosynthesis pathway and lay a foundation for further study of the environmental factors associated with wax regulatory networks in apple.

Genome-wide identification and comparative analysis of genes encoding AAPs in apple (Malus × domestica Borkh.).

Amino acid permeases (AAPs) play important roles in plant amino acid transport and nitrogen metabolism. In this study, we carried a comprehensive analysis for apple genes encoding AAPs using bioinformatics and molecular biology. Eleven MdAAPs were identified by a genome-wide search and comparative genomic analysis revealed relatively conserved gene composition, transmembrane characteristics, and protein structures. Phylogenetic tree construction and analysis of the conserved motifs of MdAAPs and AtAAPs showed that AAPs can be classified into three groups (I, II, and III). We compared the promoters of the identified genes and did gene functional annotation and qRT-PCR and found a relationship between apple AAPs and nitrogen deficiency. The expression profile data implied that MdAAPs exhibit diversified distributions and functions in different tissues.

Identification and characterization of apple MdNLP7 transcription factor in the nitrate response.

Nitrogen is an essential nutrient for plant growth and development. Low utilization of nitrogen fertilizer during agricultural production causes a series of environmental problems, such as water eutrophication, soil acidity, and air pollution. Investigating the patterns and mechanisms of crop NO3- absorption and utilization therefore key to fully improving crop nitrogen utilization rates and promoting sustainable agricultural development. Apple is one of the most important horticultural crops in the world. Its nitrogen demand by apple during the growth period is very high, but few studies have been performed on apple genes, that regulate the NO3- response. Here, we found that the apple transcription factor MdNLP7 promoted nitrogen absorption and assimilation by activating the expression of MdNIA2 and MdNRT1.1. MdNLP7 also regulated H2O2 content by increasing catalase activity, which may also influence nitrate utilization. Our findings provide insight into the mechanisms by which MdNLP7 controls nitrate utilization in apple.